The present invention relates to the fabrication of components made from superalloys, specifically to a method of bonding superalloys at an interface.
Superalloys are employed in articles such as gas turbine hot section components, high temperature heat exchangers and high temperature fluidic circuit devices because they exhibit high strength at elevated as well as ambient temperatures. Typical superalloys are nickel-based superalloys (such as INCONEL 718, INCONEL 617 and RENE 80) or cobalt-based superalloys (such as HASTELLOY X, X-40 and FSX-414). Iron-based superalloys (such as V-57) are also common. Frequently, superalloy components intended for use in heat exchangers and fluidic circuit devices having finely detailed internal passages must be bonded together using specialized diffusion brazing alloys.
Diffusion brazing relies on the solid-state diffusion of atoms across an interface of the joint between the brazing alloy and the base metal. It necessarily follows that the diffusion brazing alloys are formulated to complement the base material of the parts being joined. Diffusion brazing alloys are thus generally nickel, iron, or cobalt-based alloys, depending on the composition of the base metal, combined with one or more melting point depressants such as boron or silicon. Brazing compounds thus have a composition similar to the base alloy but a melting point that is below that of the base metal. Brazing compounds are typically provided in the form of a powder, paste, or thin foil. The bonding of a joint is effected by placing the braze material on the joint and heating the joint to a temperature above the melting point of the brazing alloy but below the incipient melting point of the base alloy. The brazing material is drawn through capillary action into the joint and, upon cooling, forms a strong metallic bond across the joint.
Brazing is not without its disadvantages. Where the parts being joined have extremely finely detailed features such as grooves or passages, the brazing alloy often wicks into and partially or completely obstructs these features. For example, the cooling panels in the transition ducts of an advanced high temperature industrial gas turbine have small cross-section cooling passages. The cooling panels are conventionally manufactured by milling a series of channels in a superalloy sheet and brazing a superalloy cover sheet over the milled channels. Conventional process controls have proved ineffectual in preventing the braze alloy from wicking into the cooling passages resulting in a high incidence of rejected parts Similarly, fluidic circuits are often composed of up to several score or more of laminated sheets 0.050 inch, 0.020 inch or even thinner. Braze alloy wicking into even one of the fine passageways of a fluidic circuit device renders the entire assembly useless.
Another disadvantage of brazing is that, because the braze alloy has a lower melting point than the surrounding base material, in extremely high temperature applications, the braze alloy will soften at a temperature lower than that of the surrounding part. The temperature limitation of the braze alloy therefore limits the operating temperature of the whole assembly. Additionally, where heat exchangers or fluidic circuits are handling highly corrosive working fluids such as the liquid sodium coolant used in fast breeder reactors, often the resistance of the brazing alloy to chemical attack is inferior to that of the base material, which limits the useful life of such devices. Post brazing heat treating improves to some extent the properties of brazed joints by diffusing the melting point depressants out of the brazed joint and into the surrounding base metal, however it is not possible to completely eliminate the braze alloy from the joint. Some interfacial residue always remains.
What is needed therefore is a method of diffusion bonding superalloy components without the use of brazing alloys.